Avalanche Photodiodes are highly sensitive photonic detectors. Avalanche photodiodes are reverse biased solid state devices with high electric field. When a photon of sufficient energy is absorbed in the device where the field exists, an electron-hole pair is generated under the influence of this field, resulting in photocurrent. If this field is sufficiently high, one of the carriers accelerates and gains sufficient energy to collide with the crystal lattice and generate another electron-hole pair. This process is known as impact ionization. The electron or hole can accelerate again, as can the secondary electron or hole, and create more electron-hole pairs, resulting in an avalanche. This process is called avalanche multiplication. When the avalanche photodiode is biased below breakdown, the rate at which electron-hole pairs are generated is lower than the rate at which they exit the field and are collected. Therefore each absorbed photon creates on average a finite number of electron-hole pairs, proportional to the bias, which is the gain of the avalanche photodiode. When the avalanche photodiode is biased above breakdown voltage, this mode of operation is called Geiger mode.
Geiger-mode avalanche photodiodes are biased above the breakdown voltage and thus have very high electric field. In this mode, when a photon is absorbed in the high field region, the electrons and holes multiply faster by impact ionization then they can be extracted, therefore the avalanche is self-sustaining and the gain is infinite, until the avalanche is stopped. In order to detect another photon or carrier event, the avalanche must be stopped or quenched. Once avalanche is detected, it is typically quenched by momentarily lowering the bias below breakdown voltage and then increasing it back to above breakdown, after the avalanche is quenched.